Electron discharge tube and circuit arrangement



Wm 18, 1936. K. STEIMEL. zmmm ELECTRON DISCHARGE TUBE AND CIRCUIT ARRANGEMENT Filed Jan. 8, 1934 IN VEN TOR K451 STEM/EL A TTORNEY Patented Feb. 18, 1936 UNITED STATES PATENT OFFICE ELECTRON DISCHARGE TUBE AND CIRCUIT ARRANGEMENT Germany Application January 8, 1934, Serial No. 705,674 In Germany January 9, 1933 3 Claims.

The present invention is concerned with a special embodiment of an electronic tube having a cathode or filament, a plate or anode, and four interposed grids (hexode tube) which is par- 5 ticularly suited for the purposes of gain regulation or automatic volume control.

It is known in the art that the amplification of a screen-grid tube which contains a cathode, a plate, a screen grid and a further grid electrode between the screen grid and the plate (hereinafter to be called a braking grid) is regulable by varying the brake-grid potential; the more negative the braking grid is biased, the smaller or less marked is the slope of the plate-current characteristic referred to the control grid voltage, and the lower also the amplification. This, for instance, allows of regulating the volume as a function of the input amplitude. In radio frequency amplifier stages, when using this method, this obstacle is encountered, namely, that the internal resistance of the tube decreases considerably just inside that range of the braking grid voltage in which an appreciable influence or action upon the slope occurs. This circumstance may be explained by that in front of the braking grid a space-charge consisting of electrons is built up which, in reference to the plate, plays the part of a virtual cathode. Between the plate and the said virtual cathode the grid electrode herein called the braking electrode, is active, and the internal resistance R1 governing and decisive for the plate current can be determined in a similar manner'as for a triode tube in accordance with this formula Rz S D=1, wherein S is the slope of the plate-current characteristic referred to the braking grid voltage and lies around the order of magnitude of 1 milliampere volt, and D, the transgrid action of the braking grid, in regard to which certain limitations are imposed inasmuch as with closely spaced grid wires too little plate current would flow. Wherefore in actual practice the internal resistance (R1) has a value such that it falls substantially below the impedance of a fiy-wheel or tank circuit contained in the plate circuit. Adaptation by the provision of a transformer, however, occasions a loss in gain.

Now, the basic idea underlying the invention is to obviate this difficulty by disposing between the so-called braking grid and the plate a further screen grid.

In the drawing Figs. 1 and 2 illustrate diagrammatically the tube construction according to the invention. Fig. 3 shows one form of the grid structure of the grid G3 in Figs. 1 and 2. Fig. 4

shows a modification of the invention utilizing a different electrode structure for G3 of Figs. 2 or 3, and Fig. 5 illustrates a circuit utilizing a tube according to the invention for automatically controlling the volume.

In Figs. 1 and 2, the electrodes are disposed as follows: the cathode K, directly around the cathode the control grid G1, then the first screen grid G2 which shields the control grid capacitively in reference to all other electrodes, next the braking grid G3 followed by a second screen grid G4 whose transgrid action across the braking grid should be of an order of magnitude of 10% or less and finally plate A, which, for the purpose of obtaining a high internal resistance possesses a comparatively small transgrid action 5%) through the second screen grid. This second screen grid need not furnish complete capacitive shielding between the braking grid and the plate. If dimensions have been chosen appropriately, the two screen grids inside the tube may be interconnected as at c for example, so that one terminal pin is saved. The connection may be established either inside the vacuum vessel or in the base of the tube.

There are a number of factors in reference to the formation and construction of the third or braking grid G3. If this grid is wound perfectly homogeneously, then the space-charge is built up in a definite form and the virtual cathode arises in a precise and pronounced form. Actual experiments have shown that in connection therewith, in the presence of a certain braking-grid potential, there arises unstcadiness in regard to the slope changes. these, and with a view to increasing the steady range of regulation, it is recommendable to wind the braking grid in a non-homogeneous manner, say, in such a way that the distances between successive grid wire turns are alternately made large and small, as shown for example at a. and b of Fig. 3. Another way and means to circumvent this difficulty which also leads to a reduction of the virtual cathode consists in that the braking grid G3 is made with an oval or elliptic crosssection While being disposed in an otherwise radial-symmetric or circular electrodev system as shown in Fig. 4 or else the grid may be wound upon a cylindrical surface eccentrically disposed with the remaining electrodes which are also cylindrical.

With a tube of such construction, a very suitable volume control is feasible, and Fig. 5- illustrates a circuit utilizing a tube according to the invention for automatically controlling the vol- In order to eliminate or mitigate ume. The tuned circuit I resonant to the incoming alternating voltage is connected between grid G1 and cathode. The plate circuit includes the resonant or tank circuit II from which the amplified alternating potential is taken ofi, then further amplified, if necessary, and finally rectified in the diode D. Connection of the audio frequency or last stage amplifier is eifected at terminals d and e. Direct voltage resulting from the rectification, which is governed by the intensity of the carrier wave of the modulated radio frequency current, is derived across the resistance R by way of the filter unit R1, C1, and impressed upon the braking grid G3 with such polarity that, with a stronger carrier wave the braking grid becomes more markedly negative. Hence, in this manner automatic volume control is insurable. The internal resistance of the tube is high enough in order that in the plate circuit direct adaptation of a fiy-wheel or resonant circuit may be feasible so that the full amplification of the tube may be realized.

Recourse could further be had to a combined regulation scheme in that at the same time the biasing voltage of the first grid may be regulated in any known manner. Hence, the first and the third grids are impressed with the regulatory biasing potential, whereas the radio frequency oscillations are impressed only upon the first grid. In this case the first grid would be designed preferably for variable transgrid action (variable mu) in order that the plate-current characteristic may have a logarithmic shape.

What is claimed is:

1. An electron discharge tube comprising a cathode, cylindrical control grid, screen grid and anode electrodes concentrically arranged about the cathode in the order named, and an elliptically-shaped auxiliary electrode interposed in the space between the screen grid and the anode.

2. The invention defined in claim 1 wherein a second screen grid is interposed in the space between the elliptically-shaped auxiliary electrode and the anode.

3. The invention defined in claim 1 wherein a second screen grid is interposed in the space between the elliptically shaped auxiliary electrode and the anode and said first and second screen grids are electrically connected together inside the tube.

KARL STEIMEL. 

